1. Field of the Invention
The present invention relates to an electrode particularly suited for use in electrolytically plating non-continuously conductive surfaces.
2. Description of the Prior Art
Electrolytic plating is often used to deposit a layer of conductive material, often copper, onto one or more conductive surfaces of a part. Since electrolytic plating relies on passing an electrical current through an appropriate plating solution, two separate electrical connections of opposite polarity must be made to the solution. As soon as current flow is initiated between these connections, (positively charged) metallic, e.g. cupric, ions flow away from the positive connection and towards the negative connection. Since the latter connection provides a source of free electrons, each cupric ion completes its valence shell at the negative connection. When this occurs the cupric ion ceases to remain ionized and thus precipitates out of solution. As a result, a layer of copper is deposited on substantially all the submerged surface area of the negative connection. Consequently, the part to be plated is typically attached to the negative connection and submerged in the plating solution. In effect the part becomes an extension of the negative connection and as a result is coated with copper during the plating process. Generally, an electrical connection is made to the part by means of a suitable electrode.
As long as the part to be plated has continuous conductive surfaces, such as for example a flat conductive sheet or sphere, the electrode can be rather simple and, in fact, is often nothing more than a suitable wire having a single terminal that is mechanically and electrically connected to the surface(s) to be plated. However, oftentimes the conductive surface is non-continuous, i.e. the surface possesses isolated conductive regions surrounded by insulating regions. For this latter situation, a single terminal will not suffice because, no matter where the terminal is affixed to the surface, some of the conductive regions will not receive any current flow and hence will not be plated.
This problem is particularly acute whenever a surface of a double-sided printed circuit board, having a number of narrow conductive traces on each side, is to be plated. In this situation, one well known solution involves the use of a so-called point-contact type electrode. Specifically, this electrode has a plurality of pins, each of which abuttingly presses against and thus establishes electrical contact with a separate conductive region existing on one side of the board in order to provide a sufficient amount of current to plate the second side. Furthermore, each of these conductive regions includes a plated-through hole which forms an electrical connection between two oppositely situated conductive regions which in effect sandwich the hole. As a result, plating current is routed from all the pins abutting against one side of the board, and through the conductive regions existing on that side as well as through the plated-through holes, to the conductive regions situated on the second side. After the second side has been plated to a sufficient thickness, the position of the board is reversed so that the second side of the board abuts against the pins thereby routing plating current to the first side and allowing a layer of copper to be deposited onto the conductive regions situated there.
Unfortunately, point-contact electrodes have several major drawbacks. First, point-contact assemblies are very bulky and cumbersome to use. Moreover, to ensure that a solid electrical contact is made between each pin and a side of the board, a spring is attached to each pin which forces it against a side of the board. However, to guarantee that all the springs will function properly, all the pins must be spaced apart by a certain minimum distance. This, in turn, disadvantageously limits the density of the pins and, hence, the resolution (minimum width) and, to a lesser extent, the orientation of the conductive traces that are to appear on the printed circuit board. In addition, point-contact electrodes are usually custom made, i.e. with the pins appropriately situated, to suit the particular layout of traces on the board to be plated. As a result, considerable time and expense are often disadvantageously consumed in fabricating an appropriate point-contact electrode.